1. Field of the Invention
The invention relates to integrated circuits, and more particularly to a method and structure for optimizing latchup robustness in integrated circuits.
2. Background Description
As electronic components are becoming smaller and smaller along with the internal structures in integrated circuits, it is becoming easier to either completely destroy or otherwise impair electronic components from latchup. Latchup is when a pnpn structure transitions from a low current/high-voltage state to a high current/low-voltage state through a negative resistance region (i.e., forming an S-Type I-V (current/voltage) characteristic).
Latchup is typically understood as occurring within a pnpn structure, or silicon controlled rectifier (SCR) structure. Interestingly enough, these pnpn structures can be intentionally designed, or even unintentionally formed between structures. Hence, latchup conditions can occur within peripheral circuits or internal circuits, within one circuit (intracircuit) or between multiple circuits (intercircuit).
Latchup is typically initiated by an equivalent circuit of a crosscoupled pnp and npn transistor. Parasitic npn and pnp transistors associated with an inverter, with a base and collector regions cross-coupled, current flows from one device leading to the initiation of the second (“regenerative feedback”). These pnp and npn elements can be any diffusions or implanted regions of other circuit elements (e.g., P-channel MOSFETs, N-Channel MOSFETs, resistors, etc.) or actual pnp and npn bipolar transistors. In CMOS, the pnpn structure can be formed with a p-diffusion in an n-well, and an n-diffusion in a p-substrate (parasitic pnpn). In this case, the well and substrate regions are inherently involved in the latchup current exchange between regions.
The condition for triggering a latchup is a function of the current gain of the pnp and npn transistors, and the resistance  between the emitter and the base regions. This inherently involves the well and substrate regions. The likelihood or sensitivity of a particular pnpn structure to latchup is a function of spacings (e.g., base width of the npn and base width of the pnp), bipolar current gain of the transistors, substrate sheet resistance and spacings (substrate resistance), the well sheet resistance and spacings (well resistance), and isolation regions.
Latchup can be initiated from internal or external stimulus. For example, latchup is known to occur from single event upsets (SEU). Single event upsets can include terrestrial emissions from nuclear processes, and cosmic ray events, as well as events in space environments. Cosmic ray particles can include proton, and neutron, gamma events, as well as a number of particle types that enter the earth atmosphere. Terrestrial emissions from radioactive events, such as alpha particles, and other radioactive decay emissions can also lead to latchup in semiconductors. Also, latchup can occur from voltage or current pulses that occur on the power supply lines, such as VDD and VSS. Transient pulses on power rails (e.g., substrate or wells) can trigger latchup processes, as well.
Latchup can be initiated by negative transient on the VDD which can lead to a forward biasing of all the n-diffusions and nwell structures and electron injection throughout the semiconductor chip substrate. This produces a “sea of electrons” injected in the chip substrate. Equivalently, a positive transient on the VSS can lead to hole injection, and forward biasing of the substrate-well junction providing a “sea of holes” event. Latchup can also occur from a stimulus to the well or substrate external to the region of the thyristor structure from minority carriers.
In internal circuits and peripheral circuitry, latchup are both a concern. Latchup can also occur as the result of interaction of the ESD (electro static discharge) device, the I/O off-chip driver and adjacent circuitry initiated in the substrate from overshoot and undershoot phenomenon. These can be generated by CMOS off-chip driver circuitry, receiver networks, and ESD devices.
In CMOS I/O circuitry, undershoot and overshoot can lead to injection in the substrate. Hence, both a p-channel MOSFET and n-channel MOSFET can lead to substrate injection. Simultaneous switching of circuitry where overshoot or undershoot injection occurs, leads to injection into the substrate which leads to both noise injection and latchup conditions. Supporting elements in these circuits, such as pass transistors, resistor elements, test functions, over voltage dielectric limiting circuitry, bleed resistors, keeper networks and other elements can be present leading to injection into the substrate. ESD elements connected to the input pad can also lead to latchup. ESD elements that can lead to noise injection, and latchup include MOSFETs, pnpn SCR ESD structures, p+/n-well diodes, nwell-to-substrate diodes, n+ diffusion diodes, and other ESD circuits. ESD circuits can also contribute to noise injection into the substrate and latchup.
In a semiconductor chip environment, there exists a plurality of different stimulus as well a plurality of circuit functions. Peripheral circuits comprise, for example, ESD networks, transmitter and receiver networks, system clocks, phase lock loops, capacitors, decoupling capacitors and fill shapes. Internal circuits can consist of DRAM memory, SRAM memory, gate arrays, and logic circuitry. In this complex environment, the latchup event can be an interaction of intercircuit interaction or intracircuit interactions.
Additionally, the interaction of the different circuits can lead to an initiation of a primary latchup event followed by a secondary latchup event. Since the circuits in a complex chip are coupled through the substrate, well, and power rails, circuit blocks and elements within a circuit block can be interactive.
Moreover, latchup vulnerability is a function of the macroscopic local temperature in a region where latchup occurs. Certain regions of a functional chip are hotter than others with a high local temperature. Regions of a chip where the chip is hotter are more likely to initiate latchup concerns. The reason this is true is that the parasitic bipolar gain increases with increased temperature. Additionally, the diffusion coefficient is a function of temperature leading to longer diffusion lengths in the region of higher temperatures in a semiconductor chip.
For external latchup issues, there is two concerns. First the intrinsic latchup parasitic structure is more sensitive to the increased temperature leading to a higher risk of latchup. Second, the diffusion of minority carriers to the triggerable network is easier as the temperature increases as a result of the higher diffusion process. Hence it would be an advantage to place these hotter circuits farther from regions of overshoot and undershoot injection.
Additionally, latchup is a higher risk in regions where the incoming voltage exceeds the native voltage. Regions of semiconductor chips that provide programmable NVRAM power pins which receive voltages that far exceed the native voltage are sources for latchup concern. Mixed voltage offchip driver circuits, mixed voltage ESD networks, mixed voltage receiver networks, mixed voltage ESD power clamps all have higher power supply voltages than the native power supply voltage of the chip. Placement of circuit blocks and circuit functions with a high density of pnpn devices, or weakly robust networks that are near the high voltage supply are more vulnerable to trigger latchup in a chip.